1. Field of the Invention
This invention relates to lighting fixtures and more specifically to lighting fixtures including a reflector having sharp cutoff characteristics.
2. Description of the Prior Art
Lighting fixtures housing a high intensity gaseous discharge (HID) lamp of appreciable wattage (e.g., 1000 watts) are used in a wide variety of applications. Many of these applications are situations where the light from the fixture spreads and provides general illumination over a wide area.
For example, a fixture in the middle of a parking lot may conveniently project light downwardly equally over a 360-degree area. By contrast, however, it readily may be recognized that it is desirable to provide light from a fixture near the edge of the lot adjoining a building having windows downwardly and outwardly over only a 180-degree area. It is desirable that light from a corner-located fixture be directed downwardly over only a 90-degree area. And, in all of the applications referred to above, it is undesirable to project light vertically above an angle that would be a nuisance from a location across the street.
Therefore, it is common to design light fixtures or luminaires of various configurations to cut off light from projecting or emanating from such fixtures beyond a certain angle or angles. But, it may be recognized that beyond the designed cutoff angle, not all of the light is cut off, only a high percentage of such light. A luminaire light distribution is designated as being cut off in accordance with IES standards when the candlepower per 1000 lamp lumens does not numerically exceed 25 (2.5%) at an angle of 90 degrees above nadir (horizontal) or 100 (10%) at a vertical angle of 80 degrees above nadir.
One popular design of luminaire used to achieve a high degree of lateral and a certain degree of vertical cutoff is an asymmetrical fixture. In such a fixture the lamp reflector usually includes side reflectors substantially parallel to the aiming axis for cutting off lateral reflections. The curvature of the remaining reflector (i.e., rearward to the lamp bulb) is then at a large arcuate angle with respect to the aiming axis on one side when compared to the arcuate angle on the other side, providing a wider opening on the first side than on the second side. Whether a light fixture which is asymmetrical can be characterized by a discussion to the standard cutoff terminology mentioned above, or with respect to aiming angles, the terminology that is common in a discussion of floodlight characteristics, it is recognized that the sharpness of cutoff on an asymmetrical fixture can be very important.
One of the reflector shapes most favored for a sharp cutoff reflector is the parabolic shape. This is because the light from the focal point of such a reflector is reflected outwardly from the reflector surface parallel to the aiming axis, which is also the long axis of the reflector parabola. A wider than parabolic arcuate spread would project a wider light dispersion and a narrower than parabolic arcuate reflector spread would cause cross light reflections, also resulting in a wider light dispersion. Most prior art asymmetrical fixtures generally have one side which is parabolic but one side which is not, the rear reflectors blending into a continuous curve. Hence, light from at least one portion of the rearward reflector is not parallel to the elongate fixture axis.
Louvers and other light restrictions have also been employed in the prior art to provide means for limiting the amount of light in one or more directions while permitting light to emanate in one or more other directions. Such louvers, however, also sharply decrease the overall efficiency of light production. Another technique for similarly restricting light is the darkening of certain reflective surfaces when compared to the specular treatment of other surfaces. Obviously, such treatment also decreases light efficiency.
It is therefore a feature of the present invention to provide an improved, highly efficient asymmetrical lamp fixture including a parabolic reflector segment that has an extremely sharp light cutoff in one direction.
It is another feature of the present invention to provide an improved, highly efficient asymmetrical lamp fixture having a shield located to enhance cutoff without impairing efficiency, as in the case with prior art louvers.
It is yet another feature of the present invention to provide an improved, highly efficient asymmetrical lamp fixture including a window opening at an angle to the elongate axis of the fixture to improve the direction of reflector light but at a sufficiently large angle with respect to the elongate axis so as not to appreciably degrade or impair the light emanating from the fixture.